1. Field of the Invention
The present invention relates to peripherals comprising a serial port for connecting to a master device, like a computer or a hub, via a serial link.
2. Description of the Related Art
The present invention more particularly relates to peripherals comprising a serial port of the USB type (Universal Serial Bus), for connecting to a master device.
A serial port of the USB type comprises four connection terminals, that is two differential data transmission terminals, and two positive and negative power supply terminals able to supply continuous current to the peripheral.
Usually, a master device comprises means for detecting the connection and disconnection of a peripheral device of the USB type. This detection function does not exist on the side of a peripheral of the USB type. However, a peripheral can be connected to a master device with one and/or the other transmission links faulty, so that the peripheral is powered by the USB link but cannot communicate with the master device. If it is a secured device, like microprocessor cards memorizing secret information, fraudsters can try to discover these secret information by powering on the card and taking measures on the data transmission terminals.
It is therefore desirable to be able to adapt the operating mode of the peripheral depending on whether it is connected to a master device, and particularly to determine whether the peripheral can securely communicate through the serial link.
The detection by a peripheral of a master device connected to the peripheral via a USB link sometimes raises several issues.
First, the USB standard 2.0 provides three data transmission speeds, that is a Low Speed of 1.2 Mb/s, a Full Speed of 12 Mb/s and a High Speed of 480 Mb/s. The master device must be able to detect the transmission speed used by the peripheral. However, according to the USB standard, the selection of one of these transmission speeds is performed by the master device according to some features of the serial port of the peripheral. In particular, a peripheral of the USB type comprises a pull-up resistor connectable to one or the other data transmission terminal of the USB port, according to the transmission speed compatible with the peripheral. This speed is determined by the master device by detecting whether one or the other positive or negative data transmission terminal is connected to this resistor. If this resistor is connected to the positive transmission terminal, the USB port of the peripheral operates at full speed. If this resistor is connected to the negative transmission terminal, the USB port of the peripheral operates at low speed. At last, if this resistor is not connected to any transmission terminals, the peripheral operates at high speed.
Next, the USB standard imposes voltage levels on data transmission terminals. Particularly, the voltage must not simultaneously exceed 0.8 Volt on both data transmission terminals.